[Comment] Risk of Zika-related microcephaly: stable or variable?

As unexpected as the epidemic of microcephaly was, the Brazilian Government immediately set up a special notification system. By December, 2015, 3174 suspected cases of microcephaly were reported (more than 1000 in one state in northeast Brazil1). Zika spread rapidly in Latin America. We braced ourselves for a vast international epidemic of Zika-related microcephaly; but when it did not happen we asked ourselves why. Were the numbers an artifact of over reporting?2 Were they real, did cofactors modify the risk given Zika virus in pregnancy,3 or was it due to something else?